The invention relates to an ink jet printing apparatus. More particularly, the invention pertains to a method and apparatus for enabling an ink jet printer to obtain uniform and clear printing.
Referring to FIG. 8, one current type of a rotary ink jet printer will be explained. In this embodiment, a paper cassette 1 is set vertically, and is pressed from the backside against a pick-up roller 2 by a pressing body (not shown). When the pick-up roller 2 is rotated by a motor (not shown) driven by an electric signal, a sheet of paper 3 is drawn and conveyed between an insert roller (not shown) and a presser roller 4. The paper 3 is further passed through a inlet rail (not shown) and is conveyed between an outer shell 5 and an inner-shell 6. The paper 3 is conveyed upward by tabs 9a, 10a provided on endless synchronized belts 9, 10 respectively.
The end-less synchronized belts 9, 10 are driven by drive-pulleys 7a and 7b, 8a and 8b, respectively. The drive pulleys 7a, 7b, 8a, 8b are driven by a paper feed motor 17 via belts or gears. While the paper is conveyed upward, an ink jet print head 11 rotates and jets ink through an orifice 13. The print head 11 is rotated by a motor 12 and provides a plurality of nozzles at its periphery and each nozzle provides an orifice 13. Thus, the printing on the paper 3 is conducted.
When the upper end of the paper 3 reaches a deflector 15 provided at a paper guide post 14, the lower end of the paper 3 is released from the synchronized belts 9, 10, turns out and drops into a paper tray 16.
In such an ink jet printer, when ink 19 (see FIG. 6) is left unjetted more than about eight hours, the ink 19a located near the orifice 13 evaporates slowly and can become more viscous. As a result, this can cause non-uniform printing and degrade printing quality. To solve such problems, Japanese patent publication No. 61-56109 has suggested that when one of the nozzles is left in an unjetted condition for more than a predetermined time period, all nozzles are purged of the ink to secure uniform printing. The purging procedure is controlled and conducted automatically by electric signals.
Further, in such ink jet printing apparatuses wherein solid ink is used and melted by heat during printing, it is possible that bubbles are formed in-between the pass-way from the ink container to the orifice since the ink volume changes as the heater turns on and off. Thus, uniform, clearly dotted printing can not always be conducted. In order to solve this problem, bubbles in the pass-way must be removed. To do so, air is supplied to press the ink and the piezo element works to purge the ink against a gutter 11a (FIG. 8).
According to the above-described type of ink-jet printer, when one nozzle is left unjetted for more than a predetermined time period, all nozzles jet the ink regardless of the printing requirement. Such an abrupt mechanical action can render a feeling of uneasiness to the operator and waste ink. Besides, when the ink is purged, a droplet of the ink can stick around the orifice, as FIG. 7A shows. In this case, the printing position can become deviated and the printing quality degraded.
It is therefore an object of the invention to eliminate the afore-mentioned disadvantages of ink jet printers.